The present invention concerns power stages with multiple switch nodes and more particularly to a keep-alive circuit for the same.
FIG. 1 shows a totem-pole switching power stage 100 that includes a first, active switching device (“switch”) 110 and a second switching device (“switch”) 120 that are in electrical communication at a common node, referred to as a switch node 115. The first, active switch 110 can be, for example, a mechanical relay, a bipolar junction transistor (BJT), a field effect transistor (FET), an insulated gate bipolar transistor (IGBT), and the like. The second switch 120 can be active like the first switch 110 or passive, e.g., a diode and the like.
The first, active switch 110 is connected to a first power supply (“voltage”) rail 130 at a first node 125 and the second switch 120 is connected to a second voltage rail 140 at a second node 135. The voltage potential at each node 125, 135 differs. As a result, when one of the switches is closed, i.e., ON, voltage at the common node 115 approximately equals the voltage on the node of the closed switch. Thus, when the switches 110, 120 are driven in a complementary fashion, the voltage at the common node 115 rapidly changes between approximately equal to the voltage at the first node 125 to approximately equal to the voltage at the second node 135. Typically, during complementary switching, the voltage at the common node 115 spends very little time between the voltages at the first node 125 and at the second node 135.
Generally, due to lower cost, wider availability, and higher performance, circuit designers prefer to use an NPN transistor and/or an N-channel MOSFET as a first, active switch 110 when practical. Problematically, in contrast with PNP transistors and P-channel MOSFETs, in order to provide drive signals to the control terminals, e.g., gates, bases, and the like, NPN and N-channel transistors require a voltage that exceeds the voltage potential at the node 125, 135.
Conventionally, circuit designers address this by providing an external drive voltage and/or by using bootstrap capacitors. The former solution requires increased voltage generation circuitry, which affects system cost and further adds to the voltage stress applied to the drive circuitry. Voltage stresses on the drive circuitry, however, can be reduced by generating a floating supply referenced to the common node 115. Consequently, cost is the major drawback associated with providing an external drive voltage.
In contrast, bootstrap capacitors offer a cost effective solution that also controls voltage stress in the drive circuitry. Referring to FIG. 2, bootstrap capacitors 16, 18 are electrically coupled to the common node and to drive circuitry 11, 17 of the first, active switches HS1, HS2. The bootstrap capacitor 16, 18 is further electrically coupled to a power source BP. During operation, the bootstrap capacitor 16, 18 powers the drive circuitry 11, 17 of the first, active switch HS1, HS2 when the first, active switch HS1, HS2 is closed, i.e., ON.
The power source BP further recharges the bootstrap capacitor 16, 18, when the first, active switch HS1, HS2 is open, i.e., OFF, and the complementary second switch LS1, LS2 is closed, i.e., ON. As a result, for bootstrap capacitors 16, 18 to work, each switch HS1, LS1 (HS2, LS2) of the complementary switching pair SW1 (SW2) must be ON for some portion of the total switching cycle and must be OFF for some portion of the total switching cycle. Otherwise, the bootstrap capacitor 16, 18 is not able to recharge.
Disadvantageously, the efficiency of the power stage is reduced when the complementary switches HS1, LS1 (HS2, LS2) at a switch node SW1 (SW2) are alternately turned ON and OFF during a cycle. Indeed, ideal operation of a power stage may require continuous operation, i.e., no switching, of the complementary switches HS1, LS1 (HS2, LS2) in the switching pair SW1 (SW2).
Accordingly, it would be advantageous to provide a device that recharges bootstrap capacitors without switches to enable continuous operation of complementary switches at a switch node. Moreover, it would be desirable to provide a device for use in multiple switch node power stages having N-channel FET or NPN transistor switches that prevents discharge of the respective bootstrap capacitors when the corresponding N-channel FET or NPN transistor switch is operated at a duty-cycle of 100% or substantially 100%. It would also be desirable to provide a device that prevents over-voltage on the bootstrap capacitors when recharging the bootstrap capacitors.